Orifice fittings are commonly employed to measure the volume of gas flowing through a pipeline or from a well. The flowing gas is passed through an orifice of a fixed, predetermined size and the pressure drop across the orifice is measured to determine volume and, hence, total cost of the gas flowing past. Because the orifice plate is subject to wear by impacting of sand, line scale and other foreign particles in the flowing gas stream, it must be replaced at frequent intervals to ensure accuracy in measurement.
Conventionally, the orifice plate is positioned within the body of the fitting across the flow passage with seals closely surrounding it to ensure that all gas flow is through the orifice. In one type of orifice fitting the orifice disc seal ring is generally U-shaped in cross-section and is stretched over the orifice plate to be received and seal around the periphery thereof. In order to apply such a seal ring, it must be made of a soft, low modulus, high elongation material that is easily stretched without breaking or tearing. Such material is generally limited to lower pressure applications, and any attempt to use it in high pressure service, say above 720 p.s.i., may result in swelling and/or blistering failure.